Injection molded articles are used for a variety of purposes. Plastic injection molded articles are commonly made from materials such as Polyethylene Terephtholate (PET) or Polypropylene (PP). In many applications, an injection-molded container has a lid or closure heat-sealed to an open portion of the container. Often, the container has a flange, lip or other protuberance at the open end of the container against which the closure is sealed. Commonly, the closure comprises a first layer configured to enclose the container and a plastic layer coating at least a portion of the first layer that contacts the container. In many applications, the first layer contains a foil, e.g., aluminum foil, or another material that provides a gas and/or water barrier for the container opening. The plastic layer is typically the same (or similar) material as the container that is capable of forming a heat-seal with the container material. The plastic material of the container and the closure in the area where the container and plastic layer contact is then heated (by various known heating methods), often with compression, which sufficiently softens and/or melts the plastic layer and/or adjacent plastic container material to seal the lid to the container. The heat-seal process results in a heat-affected zone in the container material adjacent to the heat-seal, e.g., about 10% of the thickness of the flange.
Plastic materials such as PET and PP are gas (e.g., oxygen, nitrogen, etc.) permeable. For applications in which gas permeability is undesirable, for example, food products, medicines and products that degrade upon gaseous exposure, a barrier material or scavenger material is co-injected with the plastic material. Typically, the barrier material, such as Ethyl Vinyl Alcohol (EVOH), is injected at the interior of the PET or PP material stream, forming an EVOH interior layer embedded within an inner and outer layer of PET or PP.
This co-injection process has previously been limited to articles that are essentially symmetrical in shape due to process limitations with respect to forming the barrier layer. In addition, in order to provide an interior layer that sufficiently extends through the molded article to prevent undesirable gas permeation, the interior layer material is injected into the mold in such a manner so that it flows throughout essentially the entire mold.
However, injecting the interior layer material in this manner can cause the interior layer material to flow beyond the desired interior location. For example, the interior layer material can penetrate or breakthrough the flow front or leading edge of the inner and outer layer material. If the interior layer penetrates into the heat-affected zone of the heat-seal, delamination can occur, leading to heat-seal failure. Presently known solutions attempt to more precisely control the flow of the interior layer material, e.g., by controlling injection pressure, temperature, timing, injection location, etc., so that the interior layer flows sufficiently throughout the mold cavity without flowing beyond the desired interior layer locations. Nonetheless, remaining systemic and process variations still result in interior layer material flowing into the heat-affected zone.
Accordingly, there is a need for methods and apparatuses for forming injection molding articles having an interior layer where the interior layer material does not detrimentally extend or impinge into the heat-seal affected zone. There is further a need for co-injection molded articles containing such an interior layer, but in which the interior layer material does not detrimentally extend or impinge into the heat-seal affected zone.